I am on Nextcloud 12.0.5 now, but issue was started even at 12.0.3. At one moment authorization in the Android app became broken. I type my NC URL, type my credetials and type log in button and nothing happens. But I can connect to demo Nextcloud server. I’ll also tried the Dev application, it does not connecting too.

Guys, I still have troubles with android Nextcloud and Nextcloud Dev, but I have no problems with android ownCloud client. Also it seems trouble is only with one account, with other accounts I can log in smoothly.